1. Field of the Invention
Dual-fuel engines are typically diesel or liquid fuel engines with which up to about ninety percent of the fuel can be natural gas. Dual-fuel engines are often used to power emergency or standby generators that are associated with critical facilities such as hospitals. They are also used to drive wellhead generators that run on natural gaseous fuel being recovered from the associated well. In most applications the variable control of the liquid/gaseous fuel ratio is important. This application is directed to an open loop control of the liquid/gaseous fuel ratio with a plurality of digitally-actuated gaseous fuel valves.
2. Description of Related Art
There is a well-known need to control the liquid/gaseous fuel ratio in dual-fuel engines based on load and other factors. Ordinary diesel engines can be converted to run on a mixture of diesel and natural gas (or the like) by fumigating the input air with natural gas and injecting a reduced amount of diesel fuel into the cylinders at the top of the compression stroke. The operation of dual-fuel engines in this manner is described in U.S. Pat. No. 6,250,260 to Green entitled “Bi-Fuel Control System and Assembly for Reciprocating Diesel Engine Powered Electric Generators, incorporated herein by reference. As explained in the Green patent, as the amount of gaseous fuel is increased, the amount of diesel fuel is automatically decreased by the speed governor that controls the amount of diesel fuel injected to the cylinders at the end of each compression stroke. In order to assure ignition, typically at least ten percent of the fuel must be the liquid fuel injected directly into the cylinders. For safety, such systems have solenoid operated shut off valves for disconnecting the gaseous fuel supply.
The percentage of the total fuel which can be safely or advantageously replaced by the gaseous fuel varies with operating conditions. Thus, it is necessary to control the gaseous fuel substitution rate. Control of the gaseous fuel substitution rate has been previously done on a closed loop basis using traditional analog control loops whereby a continuously variable device is adjusted by comparing a feedback signal with a command signal and substantially continuously adjusting the output of the variable device to diminish the difference between the command signal and the feedback signal. After each adjustment, new operating conditions are compared to the desired (commanded) condition until the desired substitution rate is reached. In a typically implemented digital closed loop system, it normally takes a number of iterations to reach the desired operating condition. Even in a purely analog system, this is essentially an iterative process in which it may take on the order of tens of seconds or minutes.
In certain applications, for example, an oil well drill rig generator driver, the engine load is very cyclical and never stays at a constant operating condition for any length of time. In these applications the closed loop system for gaseous fuel substitution does not work as well as desired. Operating conditions can change faster than the control loop can make the needed adjustments; therefore the desired equilibrium at set point may never be reached.